Because there were so many white men overseas or drafted into the military, there was a large demand for laborers with a limited supply. There was virtually no immigration during the World War I years, so that source of workers had dried up too.
Those white workers left behind found that it was easier to find and to keep work, and that it paid better than years previously. We start to see the emergence of a middle class at this time, though it wasn't widespread until the 1920s. This labor shortage helped African-Americans also, in that they were able to leave the South if they wished, and received better pay than ever before (still less than whites though).
For women, their contribution in the war industries (women were drunk or absent much less than men, and production actually rose when they were in the workforce) helped them finally win the 19th amendment and voting rights.
This time period was very important to African American workers. Many lived in the south and during this time there were many floods and there was a lot of crop failure. The north was booming with industrialization so many black workers migrated north for work. This scared many southernersbecause it meant that they were losing their cheap labor force. This opened some opportunity for African Americans to start a better life in the north, however there was still a lot of discrimination against them.
Women also played a more active role in the economy during this time period. Industry was popping up everywhere and since many men were fighting in the war they needed workers to fill the positions. When the war ended and men returned, many women did lose their jobs to men.
I imagine your book has specific things you are supposed to say for this...
During this war, both women and African Americans were able to move into jobs that had been previously closed to them. However, these jobs did not remain open to them (especially to women) after the war. Most of the jobs (the best one, especially) went back to white men.
However, WWI did start a huge change in the lives of many black Americans. It started a movement to the North known as the Great Migration. Before the war, almost all blacks were rural southerners. During and after the war, the black population became much more urban and northern.